Emulsions are formed from at least two liquid phases, typically oil and water, that are immiscible in each other. In an oil-in-water emulsion, for example, the oil phase is comprised of ingredients which are substantially insoluble in water. In this type of emulsion, the oil phase is composed of droplets that are finely dispersed in the water phase. It is therefore referred to as the inner or discontinuous phase, while the water phase is referred to as the outer or continuous phase. Conversely, in a water-in-oil emulsion, the water phase is finely dispersed in the oil phase and is referred to as the inner, discontinuous, phase, while the oil phase is the outer, continuous phase. Emulsions are inherently unstable and tend to separate into their constituent phases. Emulsions must therefore include emulsifiers which help create and maintain the uniform fine dispersion of the inner phase in the outer phase, and retard or prevent coalescence of the droplets and eventual separation of the emulsion into its constituent phases.
Emulsions—both water-in-oil and oil-in-water—are typically opaque (i.e., white or pale yellow in color). Transparent emulsion systems were described, in theory, as early as the 1940s. In those systems, in order to achieve transparency, the refractive indexes of the oil and water phases had to be matched prior to formation of the emulsion. See, e.g., Clayton, Theory of Emulsions, page 153 (4th Edition, 1943). Although they had been described in the 1940's, transparent emulsion systems of this type did not become commercially available until much later.
Transparent emulsions first became commercially available in the early 1960s. However, these emulsions did not achieve their transparency by matching refractive indexes. Rather, a transparent appearance was achieved by making the size of the dispersed droplets small enough (i.e., less than about 0.08 microns) so as to be unresolvable by visible light. Such emulsions are known as microemulsions. See, e.g., Gallagher, “Microemulsion Gels: A Formulator's Guide,” Happi (February 1993). Microemulsions typically required a high content (i.e., 15-20%) of high hydrophilic-lipophilic balance (“HLB”) non-ionic ethoxylated emulsifiers. Illustrative are Score™ and Clean and Groom™ hair dressings. See, e.g., U.S. Pat. No. 3,101,300. Because non-ionic ethoxylated emulsifiers are defatting and irritating to the skin when used at high levels, products containing them were viewed as unacceptable for topical application to the skin for an extended period of time. At least one previous attempt to formulate a clear microemulsion gel without the use of ethoxylated emulsifiers was unsuccessful. Gallagher described such an ethoxylated emulsifier-free microemulsion gel as not stable, having a very high set point, becoming clouded upon aging. Gallagher, supra.
By the early 1980s—with the availability of cyclomethicone and the introduction of specialty water-in-silicone emulsifiers (e.g. dimethicone copolyols) as raw materials—the development of transparent water-in-oil emulsions that were not microemulsions and that achieved their transparency by matching the refractive indexes of the oil and water phase became commercially possible. See, e.g., Dow Corning, “Using Silicone Formulation Aids to Formulate Cosmetic Systems: Quick Start Guide” (1995). Such transparent water-in-oil emulsions had several drawbacks. For example, they have the drawbacks typically associated with water-in-oil emulsions in general, such as delayed bioavailability of water-soluble active ingredients. Because such active ingredients are in the inner phase they can only become bioavailable after the emulsion breaks down. In addition, it is difficult to make refractive index adjustments at the end of processing. Therefore, in order to achieve transparency, the refractive indexes of the oil and water phases of these transparent water-in-oil emulsions had to be matched before combining the two phases.
Transparent oil-in-water emulsions overcome the above limitations. First, they have the advantageous characteristics typical of oil-in-water emulsions. Thus, for example, water-soluble active ingredients can be easily added to the aqueous phase and are bioavailable at, or shortly after, application. In addition the refractive index of the aqueous phase can be readily adjusted (e.g., by adding water to adjust the refractive index downward, or by adding glycerin or propylene glycol to adjust the refractive index upward) after the emulsion has been formed to match the refractive index of the oil phase. It is therefore not necessary to perfectly match the refractive indexes of the two phases before formation of the emulsion.
While some transparent oil-in-water emulsions have been described in the prior art, such emulsions generally contain ethoxylated emulsifiers and have a tendency to dry the skin. Furthermore, they generally require more vigorous mixing and are more viscous so that they tend to trap bubbles, which are difficult to remove and which negatively affect the clarity of the final product. Thus, there is a need for transparent oil-in-water emulsions which are not only milder to the skin, but also easier to process.